Motion detection enhances wireless network switching

In experiments on MIT’s campus-wide Wi-Fi network, the researchers discovered that, for users moving around, the protocols could often improve network throughput — the amount of information that devices could send and receive in a given period — by about 50 per cent.

The MIT researchers — graduate student Lenin Ravindranath, Prof Hari Balakrishnan, associate professor Sam Madden and postdoctoral associate Calvin Newport — used motion detection to improve distinct communications protocols.

One is said to govern the smartphone’s selection of the nearest transmitter.

‘Let’s say you get off at the train station and start walking toward your office,’ said Balakrishnan. ‘What happens today is that your phone immediately connects to the Wi-Fi access point with the strongest signal. But by the time it’s finished doing that, you’ve walked on, so the best access point has changed. And that keeps happening.’

By contrast, he added, the new protocol selects an access point on the basis of the user’s inferred trajectory.

‘We connect you off the bat to an access point that has this trade-off between how long you’re likely to be connected to it and the throughput you’re going to get,’ said Balakrishnan.

In their experiments, the MIT researchers found that, with one version of their protocol, a moving mobile phone would have to switch transmitters 40 per cent less frequently than it would with existing protocols. A variation of the protocol improved throughput by about 30 per cent.

Another of the protocols governs a phone’s selection of bit rate, or the rate at which it sends and receives information.

Bit rate needs to be tailored to the bandwidth available, given that data can be lost when sending too much data over a weak connection. Solving that problem by keeping the bit rate slow can lead to wasted data capacity.

When a device is in motion, the available bandwidth is constantly fluctuating, so selecting a bit rate becomes more difficult. Because a device using the MIT protocol knows when it’s in motion, it also knows when to be more careful in choosing a bit rate.

In the researchers’ experiments, the gains in throughput from bit-rate selection varied between 20 per cent and 70 per cent but consistently hovered around 50 per cent.

A third protocol governs the behaviour of the wireless basestations rather than the devices that connect to them.

Ordinarily, a basestation knows that a device has broken contact only after a long enough silence. In the interim, the basestation might try to send the same data to the device repeatedly, waiting for acknowledgment and wasting time and power. With information about the device’s trajectory, the basestation can make an educated guess about when it will lose contact.

The MIT team presented its work at the Eighth Usenix Symposium on Networked Systems Design and Implementation, which took place in Boston in March.